The following account of the prior art relates to one of the areas of application of the present application, hearing aids.
Acoustic feedback occurs because the output loudspeaker signal from an audio system providing amplification of a signal picked up by a microphone is partly returned to the microphone via an acoustic coupling through the air or other media. The part of the loudspeaker signal returned to the microphone is then re-amplified by the system before it is re-presented at the loudspeaker, and again returned to the microphone. As this cycle continues, the effect of acoustic feedback becomes audible as artifacts or even worse, howling, when the system becomes unstable. The problem typically appears when the microphone and the loudspeaker are placed closely together, as e.g. in hearing aids. Some other typical situations with feedback problems relate to telephony, public address systems, headsets, audio conference systems, etc.
A particular problem occurs when the coupling conditions of a hearing aid (in particular an ITE part of a hearing aid) to a user's ear canal is different from what is intended (e.g. different from what was assumed when the hearing aid was designed and/or fitted to the person in question), e.g. because the mounting of the hearing aid in the ear canal is less than optimal or because the ear canal changes over time. The latter is e.g. the case for children. Because the ears of children grow fast, it is important with a pre-warning by a leakage detector and possibly to lower the gain depending on the detected leakage.
It is known to apply a digital loop gain estimator in a DFC system (DFC=dynamic feedback cancellation), and also to realize a digital maximum gain limiter under control of the DFC. This feature is known as a fast online feedback manager. A fast and a slow online feedback managing (OFBM) system are e.g. described in WO 2008/151970 A1. Using the fast and slow OFBM parts of such a system, a long term maximum insertion gain (IGmax) can be estimated and changes to the limits for the gain in the hearing aid can accordingly be made to avoid long term problems with a hearing aid that sounds bad or is likely to howl (e.g. due to child growth). The long term IGmax is estimated by logging fast (current) IGmax estimates provided by the DFC system and filtering them to provide a slower varying long term estimate.